Is it true that a neuron transfers information in the form of an electrical impulse?

6.3.5 Conduction Blocks

The propagation of an electrical impulse can be disturbed by a block along its normal conduction pathway. The block causes depolarization and repolarization to become abnormal, disturbing the function of the heart. One conduction block is related to the AV node and makes the electrical connection between the atria and ventricles abnormal to various degrees. The severity of the AV block is graded from minor, when all impulses are conducted with delay, through moderate, when some impulses do not reach the ventricles, to complete, when no impulses are conducted. A complete AV block is manifested by P waves and QRS complexes at two different, independent rates; the P waves are produced by the SA node, while the QRS complexes have their origin in a ventricular ectopic focus. Many other types of conduction blocks may occur, e.g., in the left or right bundle branches [18].

13.2 Electricity in Plants

The type of propagating electrical impulses we have discussed in connection with neurons and muscle fibers have also been found in certain plant cells. The shape of the action potential is the same in both cases, but the duration of the action potential in plant cells is a thousand times longer, lasting about 10 sec. The speed of propagation of these plant action potentials is also rather slow, only a few centimeters per second. In plant cells, as in neurons, the action potential is elicited by various types of electrical, chemical, or mechanical stimulation. However, the initial rise in the plant cell potential is produced by an inflow of calcium ions rather than sodium ions.

The role of action potentials in plants is not yet known. It is possible that they coordinate the growth and the metabolic processes of the plant and perhaps control the long-term movements exhibited by some plants.

Physiological principles of FES

In FES, small electrical impulses are applied to the nerves that supply the affected muscles using either self-adhesive electrodes placed on the skin or implanted electrodes on the nerve or muscle close to the motor point. The electrical current generates an electric field between the pair of electrodes (Fig. 21.1), and, with the right conditions, may induce a nerve impulse that is propagated along the nerve to the muscle, causing the muscle to contract in a manner very similar to natural contraction. Though there are obvious differences between these two delivery techniques, for this section they can be considered in the same way.

As a result, each nerve in the vicinity of this electric field may be excited. What is meant by the phrase “the right conditions” is that a nerve is required to be in an excitable state and that the level of stimulation intensity needs to be sufficient to cause excitation. It needs to be remembered that just as in normal nerve excitation, the all-or-none principle is maintained. The characteristics of the electrical pulses are therefore important: the amplitude, the pulsewidth, and frequency of electrical pulses. This perhaps over-simplifies the nature of these pulses, but is fundamental to how a train of pulses can generate a required movement useful for function.

The amplitude and pulsewidth can be regarded as being synonymous with the stimulation intensity. Below a certain level of intensity there is no muscle response because the level is insufficient to cause any nerve excitation. The point where nerves begin to become excited is called the threshold of stimulation. As the intensity increases, the response curve follows the classic S-shape, with a steep linear slope over much of the middle part of the curve, leading to a plateau, where the resulting muscle response does not increase even with an increase in intensity (Fig. 21.2). This increase in muscle response over the S-shaped curve is caused by more motor units being recruited, partly due to the electric field penetrating deeper (i.e., more nerves are in its vicinity) and partly because the intensity is now above the excitability threshold of more nerves. The plateau is at the point where no additional nerves can be excited.

The effect from the frequency of the stimulation pulses is slightly different. Increasing the frequency (i.e., reducing the inter-pulse interval) also causes an increase in force produced from the muscle, but not from exciting more motor units. The cause here is from summating contractions from the same nerve (or nerves) being excited, since increasing the frequency does not allow the muscle to return to its state of rest between pulses (Fig. 21.3). At a certain frequency, these contractions summate to the point where they become fused; this is termed tetany. For most functional activities, a fused, or smooth, contraction is required, demanding a higher frequency, however a higher frequency shortens the rest time between pulses and therefore has the disadvantage of increased muscle fatigue (Fig. 21.4). A compromise between a sustained smooth contraction and muscle fatigue must therefore be considered when selecting the stimulation frequency.

Recording setup.

Stimulation is performed by delivering a brief electrical impulse via two stimulus electrodes positioned close to the sensory nerve fiber. Similar to the recording of AEPs and VEPs, SEPs are recorded by placing electrodes over the motorsensory cortex at predefined locations. However, a number of additional electrodes are needed, and these are positioned along the nerve pathway to the cortex, e.g., on the knee and the spinal cord.

In clinical routine, SEPs are usually recorded by stimulation of three different nerves: the median nerve in the arm and the tibial and peroneal nerves which are both in the leg.

5 Electrocardiography

The constant muscular contractions of the heart during the cardiac cycle are triggered by regular electrical impulses originating from the heart's sino-atrial node (the heart's “pacemaker”). These impulses conduct throughout the heart, causing the movement of the heart's muscle. Certain diseases can produce irregularities in this activity; if it is sufficiently interrupted, it can cause death. This electrical activity can be recorded and monitored as an electrocardiogram (ECG).

Through the techniques of electrocardiographic imaging, ECG data can be mapped into a 2D or 3D image [46]. These so-called body-surface potential maps are constructed by simultaneously recording and assembling a series of ECGs. Such image data can be used to visualize and evaluate various disease states, such as myocardial ischemia, where the blood flow is reduced to a portion of the myocardium. Angiographic and CT imaging cannot provide such data. Body-surface potential maps also permit the study of ventricular fibrillation, a condition when the heart is excited by chaotic—and potentially lethal—electrical impulses.

Standard analytic methods from electromagnetics, such as the application of Green's theorem to compute the electric field distributions within the heart volume, are applied to evaluate such image data. Figure 18 gives an example. Recent work has combined sophisticated computer-graphics techniques and stereoscopic imaging to enable better localization and analysis of ECG data [47].

Presentation

The Presentation layer starts getting closer to things that humans can actually understand. Instead of electrical impulses (physical) or binary code (data link), the Presentation layer deals with standards that define actual characters and how data gets presented to devices. This layer also has definitions associated with compressing characters to require less data to represent them so that they take less time to transmit or receive on the network. Along with compression, there are also encryption standards that function at this layer. We’ll discuss encryption in a little more detail in Chapter 5 when we discuss the topic of security.

VI.B. Fraud and Money Theft

While we still use cash for transactions, much of the money that exchanges hands is actually electrical impulses and magnetic fields. Billions of dollars are transferred daily from one bank account to another by way of simple instructions to computers and transmission of electrical signals via computer networks. By either illegally obtaining access codes or by circumventing them, criminals can transfer millions of dollars from one bank account to another from a remote computer. Contrary to popular belief, the majority of online fraud and money theft is carried out by “insiders,” that is employees of the victim organizations.

RFID reader for library

There are many different types of tag readers or sensors available for library applications. Typically, the reader is responsible for generating the electrical impulse that causes the tag to be read because the tags used in libraries are usually ‘passive.’ The reader interrogates the tag, which then replies with the information stored on the tag. Some readers store the information captured from the tags, while others capture the information and immediately pass it through to the database or ILS. In a typical library, readers are configured to identify tags for purposes of circulation, which include inventory management and theft control. Readers in the library are either fixed or handheld and are located in the library as listed below (Boss, 2007):

Tagging station is used to program the tags to be attached to the library materials.

Staff workstation at circulation is used to charge and discharge library materials.

Self-check station is used to check-in and checkout library materials without staff assistance.

Book-drop station is used to automatically discharge library materials and reactivate security.

Sorter and conveyor automated system for returning material to proper area of library.

Sensor gates are used to verify that all material leaving the library has been checked out.

Inventory manager is used for inventorying and verifying that material is shelved correctly.

Figure 2.3 depicts the various functional components in the library where RFID readers can be used.

38.4.2 Neuronal imaging

Considerable interest is now being shown in neuronal imaging. This involves measuring the electrical phenomena that occur within individual nerve cells and networks of nerve cells themselves and examining how these electrical impulses propagate through the cells. Neuronal studies usually call for very fast imaging indeed because the phenomena, particularly in mammalian neurons, are very rapid. In order to achieve very high-speed imaging (in excess of a few hundred frames per second) it is essential to use a small area CCD. Although large area CCDs may be used to produce small data sets by only reading out a subarray of the device, the overheads implicit in clearing out unwanted charge make it very difficult to achieve a good frame rate. For this reason a small area CCD such as the CCD 39 from EEV, Chelmsford, UK, which has 80 × 80 pixels, is a very attractive device. In addition the CCD is available thinned, something that is especially valuable in neuronal imaging because the high frame rate means that very little light is available for each frame. At high frame rates the read noise is inevitably high (with a CCD 39 and a read rate of 500 frames per second, a typical read-out noise might be 40–60 electrons per pixel), so that the very best quantum efficiency and the lowest possible read-out noise are essential in this application. If, however, absorbance dyes are being used instead of fluorescence dyes, then plenty of light should be available and the emphasis must be on the full well capacity of the CCD since the absorbance that is needed to be measured is often a small fraction of one percent. This requires the use of a CCD with a high full well capacity, and again the CCD 39 meets this requirement well.

2.1 Introduction: signals and systems

The term signal is defined by the Oxford English Dictionary as ‘a sign (especially a prearranged one) conveying information or giving instruction: a message made up of such signs, transmitted electrical impulses or radio waves’. Radio communication was the first application of electronics at the start of the 1900s. The word ‘signal’ continued to be used as the applications of electronics expanded, so that it now refers to all changes of voltage or current in an electronic circuit. The sole purpose of all electronic circuits (other than power supplies) is to process, transmit (transfer from one place to another) and store (transfer from one time to another) information in the form of signals. So, an understanding of the fundamental principles relating to signals and their processing is essential for the study and application of all electronics, but particularly analog electronics.

Signals, or more particularly voltages or currents, are applied to circuits of interconnected components and the resulting voltages and/or currents in another part of the circuit are measured. The terminology is self-explanatory, so that the applied signal is called the input signal, or just input, or excitation and the resulting one the output or response. Note that the term response may refer to the output signal or to the ratio of the input and output signals depending on the use by the particular author. Similarly, the terminals are called input and output terminals or ports, as shown in Figure 2.1(a). In some cases, only the behaviour of such a circuit is of interest and not its configuration, components, etc. Then it is useful to treat it in more general terms as a system or a blackbox as shown in Figure 2.1(b). The word system has become virtually meaningless in the everyday language because of its widespread use, misuse and abuse. However, in the context of this text an engineering system can be defined as ‘A set of interconnected components built to achieve a desired function’.